Serum testosterone (T) concentration and urinary 17-ketosteroid (17-KS) excretion were measured under basal conditions and after stimulation with human chorionic gonadotropin (hCG) in 36 prepubertal males (10 normal children and 26 cryptorchid patients). As a function of hCG stimulation, the increase in serum T level was evident in both groups, although the magnitude of the rise was higher in the control group than in cryptorchid boys. No significant differences in serum T or urinary 17-KS were observed between patients with unilateral and bilateral undescended testis, either before or after hCG stimulation. In spite of the wide individual variations in testicular response in all subjects, the test is valuable in assessing Leydig-cell function and in prognosis for virilization at puberty.